Respiratory effects on cardiac related impedance indices measured under voluntary cardio-respiratory synchronisation (VCRS).

The respiratory effects on impedance-determined cardiac indices ((dZ/dt)min, stroke volume (SV), the Heather index (HI) and the O-wave) were measured at 100 kHz on ten normal males in the supine and sitting positions. The respiration was synchronised to 1/5th of the heart rate using voluntary cardiorespiratory synchronisation (VCRS). Digital filtering was used to eliminate breathing artefacts in the impedance signals. SV, (dZ/dt)min and HI were statistically higher during inspiration than expiration. Ensemble-averaging of (dZ/dt) signals using the R spike of the ECG as reference will lose beat-to-beat information and statistically reduce (dZ/dt)min because of the variation of RZ intervals during respiration. Z0 increased with inspiration and decreased in expiration delayed by one heart beat. The ratio of the O-wave height to (dZ/dt)min changed by 48 per cent from the beginning of inspiration to the end of expiration. Based on the timing and direction of the changes, the data imply that the traditional band impedance measurement is more closely related to the right heart event than to that of the left heart.